THE THORAX 



991 



spending ventricle, and divides into branches which ramify beneath the endo- 

 cardium, where they form part of the system of Purkinje's fibres. Ultimately 

 the terminal ramifications blend with the muscular fibres of the ventricles 

 and musculi papillares. 



The fibres of the auriculo-ventricular bundle are pale, and are furnished 

 with nerve-fibres and ganglia. The clinical significance of the bundle has 

 reference to Stokes-Adams disease, or heart-block. (Kent, His junior, Tawara, 

 and Keith and Flack.) 



Structure of the Cardiac Valves. — Each segment of the tricuspid and mitral 

 valves consists of two layers of endocardium, enclosing between them fibrous 

 tissue which is derived frbm the fibrous ring around the corresponding auriculo- 

 ventricular orifice. Each segment of the aortic and pulmonary valves also 

 consists of two layers of endocardium, enclosing between them fibrous tissue 

 which is derived from the fibrous ring around the corresponding arterial 

 orifice. The segments of all the valves are destitute of bloodvessels. 



Minute Structure of the Myocardium. — Cardiac muscle is composed of 

 transversely striated fibres, but these differ so much from ordinary striated 



Fig. 409. — Cardiac Muscular Fibres showing their Transverse Stri.*., 

 Divisions, Junctions, and Position of the Nuclei. 



voluntary muscular fibres as to require a special description. They possess no 

 sarcolemma; they branch freely, and adjacent branches unite, so that an 

 intricate network is formed by them ; and the transverse striae are fainter and 

 less regular than those of voluntary muscle. Each fibre and its branches 

 are broken up into a series of short quadrangular muscle-corpuscles, arranged 

 in rows or chains, and separated from one another by cross-marks. Each 

 muscle-corpuscle contains one nucleus, which occupies the centre of the cor- 

 puscle and the corpuscle presents longitudinal striae as well as transverse. A 

 cardiac muscular fibre, as well as its branches, is therefore a chain of muscle- 

 corpuscles, each of which has a nucleus at its centre, and there is an absence 

 of sarcolemma. 



Weight of the Heart. — The average weight of the heart of the adult male 

 is 10. I ounces, and of the adult female 8.V ounces. 



Size. — The size of a normal heart has been compared to the closed hand 

 of the individual person. 



Development of the Heart. — The heart makes its first appearance as 

 two tubes, which are hollowed out in the splanchnic mesoderm of the peri- 

 cardial region (cardiac portion of the ventral mesentery). These tubes are 



